Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions are shrinking while wafer size is increasing. Economics is driving the industry to decrease the time for achieving high-yield, high-value production. Thus, minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for the semiconductor manufacturer.
Micrometer and nanometer scale process control, inspection, or structuring is often done with an electron beam, which is generated and focused in an electron beam apparatus, such as electron microscopes or electron beam pattern generators. Electron or other charged particle beams offer superior spatial resolution compared to photon beams due to their short wavelengths.
Wafers can be inspected using a scanning electron microscope (SEM). FIG. 1 shows a conventional electron beam apparatus 100 in an SEM with an electron source 101, an electron beam optical column 102 (shown with dotted line), and a sample 103. The electron source 101 may be a thermal field emission (TFE) source. The sample 103 may be a semiconductor wafer. The electron beam optical column 102 commonly has multiple electrostatic and/or magnetic lenses and multiple apertures. The performance of an electron beam apparatus 100 is best characterized by the electron beam spot size at the sample (d) versus the beam current delivered to the sample because the former affects resolution and the latter affects throughput. Performance (d versus beam current or d=f(beam current)) is determined both by the electron source 101 and by the electron beam optical column 102.
To cover wide applications for SEM review and inspection, the beam current is varied from pico Amperes (pA) to hundreds of nano Amperes (nA). For each beam current, the optical spot size (d) at the sample should be minimized to reach highest resolution. For these reasons, in FIG. 1 a beam limiting aperture 104 is used to give a raw beam current (e.g., highest possible beam current to the sample 103), and a column aperture 106 is used to select the beam current from the raw beam current by changing the gun lens strength to move the first crossover 109 (XO1) position. The beam current is defined or characterized by the emission angle of the source, a, with which the source is optically related to the column. Given a selected beam current, a condenser lens 107 is used to select an optimal numeric aperture (NA) through the objective lens 108 focusing the beam to the sample 103. With an optimal NA (or the β in FIG. 1), the column lens aberrations and Coulomb interactions between electrons are balanced, and the total spot size is minimized. The electron beam profile in between the condenser lens 107 and objective lens 108 may be either with a crossover 110 (XO2) or with no crossovers.
An electrostatic gun for emitting and focusing an electron beam may consist of an electron source 101 (e.g., emission tip, suppressor, and extractor) and an electrostatic gun lens 105. The electrostatic gun lens 105 can include the ground electrodes and the focusing electrode in between the ground electrodes. A focusing voltage is applied on the focusing electrode. A beam limiting aperture 104, which may be grounded, can be included.
From an application standpoint, an electron beam apparatus can be used as an SEM platform with low beam currents below sub-nano Amperes, a review platform with medium beam currents in sub-nAs to nAs, or an inspection platform with high beam currents in nAs to hundreds of nAs. This can cover the physical defect inspection, hot spot inspection, voltage contrast inspection, or other techniques.
The disadvantage of a conventional electron beam apparatus is that the optical performance is optimized or limited in one of applications with narrow beam current ranges. For instance, an SEM review tool may provide acceptable performance with high resolutions in low beam current or medium beam current, but poor performance with high beam currents. In another example, an inspection tool is may provide acceptable performance with high beam currents, but poor performance with low beam currents or medium beam currents. FIG. 2 exhibits the simulation performance of the electron beam apparatus 100 in FIG. 1 showing how the spot size varies with the full range of beam currents. A TFE electron source 101 and an electrostatic gun lens (EGL) 105 in FIG. 1 are used in the simulation for FIG. 2. A low beam current electron beam platform like an SEM may have good performance with the electron beam apparatus 100 of FIG. 1, but a high beam current electron beam platform like inspection may have poor performance.
With different electron beam currents from pico Amperes to hundreds of nano Amperes, an electron beam apparatus may be widely used for semiconductor wafer critical dimension scanning electron microscopy, review, and/or inspection. Electron beam instrument developers have been seeking to combine all these applications into one machine with high resolution for each use. However, this is challenging because electron beam resolutions vary with electron beam currents. Therefore, an improved electron beam apparatus is needed.